Molded flexible polyurethane foam parts such as automotive seating, arm rests, head rests and steering wheels are produced in molds that are precoated with a release agent prior to injection of the foam-forming components. The polyurethane foam processes are quite varied with regard to the foam-forming components which can be used and the equipment, molds and processing conditions used. The mole release agent affects both the release characteristics of the molded part as well as the surface quality of the part.
Conventional mold release compositions comprise release effective substances, such as waxes or silicones, dispersed in low boiling organic solvent carriers which control film thickness and film formation. The high solvent concentration of these compositions produces significant organic emissions during the time between the release agent application and pouring of the foam components. Increased public and regulatory agency awareness of and concern about organic emissions has led to significant impetus worldwide to reduce and eliminate organic solvents from mold release compositions.
Initial attempts at solving this problem have involved the replacement of organic solvents with non-volatile carriers such as water (U.S. Pat. Nos. 4,473,403 and 4,491,607). Compared to conventional solvent containing release materials, aqueous systems may require increased evaporation times and higher mold temperatures to remove water due to extensive hydrogen bonding. In addition to unfavorable evaporation rates, water with its high surface tension is a poor film-forming solvent resulting in poor integrity of the release film. Furthermore, the low equivalent weight of water and its small molecular size make it highly reactive to isocyanate. Water remaining in the mold release composition prior to pouring results in competitive reactions between the water/isocyanate and the polyol/isocyanate. This reaction of water with isocyanate results in carbon dioxide generation which leads to surface and subsurface defects in the foam product including bubbles, blisters, voids and, if severe enough, foam collapse.
Application of entirely aqueous systems tends to be difficult. Desirable light applications allow for reasonable evaporative rates, however, insufficient levels of release effective substances are delivered to the mold to prevent parts from sticking. Heavier applications provide for sufficient release qualities for the parts, but residual water trapped within the release film matrix results in severe surface deficiencies. Furthermore, some of these systems are prone to severe build-up over time.
Aqueous mold release systems have been developed to eliminate these long evaporation rates through the substitution of considerable portions of water with low boiling alcohols. However, the competing water/isocyanate reaction is not eliminated and the use of the solvents does not alleviate volatile organic emission concerns.
Other mold release systems have been developed based exclusively on water as the carrier as described in U.S. Pat. No. 4,609,511. This patent alleges that the water/isocyanate reaction is minimized by the masking of water molecules through the effect of "lyotropic mesomorphism". Such masking of water is achieved through the combined use of release effective substances, preferably microwaxes, thickened petrolatum fractions and polysiloxanes, and suitable auxiliary masking agents, for example, emulsifiers such as ethoxylated fatty alcohols.
These water-based release agents have found limited use in higher density methylene diphenyl diisocyanate (MDI) molded polyurethane applications such as head and arm rests. When these release agents are used in lower density applications such as automobile seating, release problems occur from build-up of release agent on the parts. In addition to release problems in lower density applications, surface and subsurface deficiencies are also observed particularly in more water sensitive toluene diisocyanate (TDI) formulations.
It has been discovered by the applicants that another cause of release problems and surface defects associated with the use of water-based mold release compositions is the physical break-down and removal of the release coating from that part of the mold upon which the foaming components impinge when charged or poured into the mold. The pouring physically erodes, or "washes off", the release coating, especially when the coating is liquid at the mold pour temperature. Surface/subsurface deficiencies in the molded foam part result.